1. Field of the Invention
The present invention relates to a developing bias device of an electrostatic recording apparatus, and more particularly to an electrostatic recording apparatus for developing an electrostatic latent image formed on a record medium (e.g. a photosensitive drum) through charging and light exposing steps, with toner.
2. Description of the Prior Art
In a conventional electrostatic recording apparatus of this type, the amount of charge, the amount of light exposure or the amount of developing bias is controlled by a digital computer based on a surface potential measured so that an optimum quality of image is recorded. In such a control, if a blank exposure lamp is not lit or a potential meausure circuit is in failure, an abnormal voltage measured is applied to the computer as a potential data. If the abnormal voltage is processed, the amount of charge, the amount of light exposure or the amount of developing bias is too abnormal to carry out normal recording of image.
A D/A converter is usually used to convert the output from the digital computer to analog values. As the LSI technology develops, D/A converter chips having various types of functions are available. For example, a certain D/A converter LSI chip has four circuits which produce pulse outputs having resolutions of 12 bits, 6 bits, 4 bits and 4 bits, respectively and have duty factors proportional to input data, and 8-bit expansion output ports.
When such a chip is used, the image forming conditions such as the amount of primary charge, the amount of secondary charge, the amount of light exposure and the amount of developing bias can be controlled by a single D/A converter LSI chip and hence it is very advantageous from the viewpoints of space factor and cost.
On the other hand, in spite of the advantage of multi-output, the resolution is restricted. Accordingly, when a four-bit output is used to control the amount of developing bias, the resolution is short. Although it may be relieved by using other high order bits, another control factor (for example, primary or secondary charge) is sacrificed. Since it is desired to control the amount of primary charge and the amount of secondary discharge with rather high precision, 12 bits or 6 bits are used therefor. The amount of light exposure may be controlled by four bits because a control range is not wide and the control may be repeated several times. Regarding the amount of developing bias, if a control range of -200 V to 400 V D.C. developing bias is selected, the resolution is approximately 38 V/bit for the 4-bit output. This resolution is too coarse for the amount of developing bias to attain a sufficient function for intended stability of image.
When a micro-computer is used to digitally control the latent image forming conditions, a digital computer (control unit) which stores a program for sequence controlling the latent image formation and a digital computer (control unit) which stores a program for controlling the latent image forming conditions or the developing conditions are used with data being exchanged therebetween. However, if a failure such as a break in a connector or a cable in a data line between the units occurs, a malfunction due to the abnormal data is not detected or the malfunction is not corrected so that a proper image is not formed.
An electrostatic recording apparatus has been proposed in which a surface potential is measured by a potential sensor and a potential measuring circuit and the surface potential measured is A/D converted to control the image forming conditions such as charge and developing bias by processing it by a microcomputer. In such a system, the potential sensor and the potential measuring circuit are controlled in union with a gain of the measuring circuit being variable. Accordingly, when the potential sensor and the potential measuring circuit are replaced in union, only level adjustment of an A/D converter section is needed. They are connected to the microcomputer such that when a potential of zero volt is applied to the potential sensor the microcomputer produces a data corresponding to zero volt. For example, the level of the A/D converter is adjusted while monitoring 8-column LED cells which indicate 8-bit data. If, for some reason or other, a failure occurs in the potential sensor or the potential measuring circuit, the potential measuring circuit and the A/D converter circuit may be isolated from each other, as a tentative measure, to operate the apparatus only with the circuits including the A/D converter and succeeding stages. In this case, it is desirable for the following reasons that the A/D converted input is set to the same level as that defined by the zero volt of the potential sensor.
.circle.1 Readjustment of the level of the A/D converter circuit is easy to attain. PA1 .circle.2 By checking the result of the control operation which is carried out under the assumption that the surface potential of zero volt was measured, the operation of the combination of the A/D converter, the microcomputer, the D/A converter and the controller can be diagnosed. (The control operation by the zero volt measurement relates to the control for the amount of light exposure and the amount of developing bias.
If a surface potential of a photosensitive member is zero volt at irradiation by a standard white plate, the micro-computer produces a standard value without requiring any decision on a control voltage because a target voltage is zero volt. The fact that the potential for determining the developing bias is zero volt means that +100 V, for example, of developing bias is produced.)
In the prior art apparatus, however, since the measuring circuit has an output impedance of 100 K.OMEGA., if the zero volt is directly applied to the A/D converter input after the measuring circuit and the A/D converter circuit have been isolated from each other, a voltage which exactly corresponds to the zero volt input is not A/D converted and the indication does not show a predetermined value. Accordingly, it is necessary to connect a resistor of 100 K.OMEGA. (of 1% grade) corresponding to the output impedance between the A/D converter input and ground by means of a clip or the like. The tentative operation of the apparatus under such a condition is unstable.
In such an electrostatic recording apparatus, a fog appears in the reproduced image or the density of the image is reduced depending on the condition of the photoconductor or the photoconductive drum and hence an image of proper density cannot be reproduced. This will be explained below with reference to FIGS. 11 and 12.
FIG. 11 shows characteristic curves illustrating a relationship between a surface potential of a photoconductive drum and an exposure quantity. A curve (A) shows a characteristic at room temperature and room humidity, a curve (B) shows a characteristic at high humidity and a curve (C) shows a characteristic after an aging test.
The quality of image adjusted under the characteristic (A) is apt to produce fog in the characteristic (B) and is apt to reduce the density in the characteristic (C). If a user wants to adjust the quality, he or she must operate an image quality selection switch which is linked to an exposure quantity dial or developing bias. Since this adjustment usually needs the adjustment of charge quantity, it is not possible for the user to adjust.
FIG. 12 shows characteristic curves illustrating a relationship between a potential and an exposure quantity when a charge voltage is controlled by dark and bright area potentials measured. The characteristics (A), (B) and (C) correspond to those of FIG. 1. While the potential control in FIG. 12 is intended to overcome the shortcoming encountered in FIG. 11, there still remain differences between the characteristic (A) and the characteristics (B) and (C). In order to prevent the fog due to the difference in the characteristics, it has been proposed to measure a white background potential to control the developing bias. However in this developing bias control system, the image density is reduced by for the following reason. In the characteristic (A) of FIG. 12, it is assumed that a potential on a photoconductor is a when a standard plate located beyond an image area is irradiated by a standard exposure quantity ES and a developing bias controlled with reference to the potential a is applied to a developing electrode to reproduce a proper quality of image. When the characteristic changes from (A) to (B), the potential under the standard exposure quantity rises to b. Since the developing bias is controlled with reference to the potential b, a difference between a dark area surface potential (E.sub.D in FIG. 2) and the developing bias potential is reduced. As a result, the fog is eliminated but the density is reduced. In the characteristic (C), the potential falls to c and the developing bias is controlled with reference to the potential C. As a result, a difference between the dark area surface potential and the developing bias potential is larger than that for the characteristic (A) and a higher density of image is reproduced.
In order to compensate for the difference in the density, the standard exposure quantity Es has to be changed in accordance with the status of the photoconductor, such as Es' for the characteristic (B) and Es" for the characteristic (C) in order to maintain the potential under the standard exposure quantity at a constant level. However, it is troublesome to carry it out manually.
In a one-component toner developing process in which a carrierless toner is used to develop the image, a jumping development process (e.g. refer to Japanese published unexamined patent application No. 55-18656) is used to attain uniform charging of toner and toned development. However, in such a one-component toner jumping development process, because of a large distance between a photoconductive drum and a developing sleeve, a characteristic curve for the photoconductive drum potential and the developed density exhibits a sharp rise as shown by a solid line (A) in FIG. 9 resulting in a low tone image. Accordingly, a high A.C. electric field is applied to the developing sleeve to reciprocate the toner between the drum and the sleeve in order to smooth the characteristic as shown by a broken line in FIG. 9 to attain a high tone developed image. The A.C. bias applied to the developing roll must have a very small waveform distortion at a frequency of 200.about.1500 Hz and an amplitude of 600.about.200 V P-P. In a conventional apparatus, a square wave generated by a multivibrator or a blocking oscillator is used to drive a transformer and an LC resonance circuit is formed by a secondary inductance of the transformer or another choke transformer to produce a sinusoidal wave of a desired frequency. As a result, a large electric power is consumed in the resonance circuit and a large exciting current to the transformer is required. As a result, a vibration noise is produced and adjustment for resonance is required. In order to vary the frequency, a large inductance or capacitance must be varied. Such adjustment is practically difficult to attain.
In a charger for charging the photoconductive drum in such a recording apparatus, a high voltage corona voltage is controlled by a closed loop constant current control system using a high gain differential amplifier. Since a high voltage transformer and a constant current control circuit in high voltage generating means are usually connected via a connector, if a feedback loop to the differential amplifier is opened by an incomplete connection of the connector, a break in a wire or a misconnection, an input in the feedback loop rises close to a power supply voltage and the high voltage transformer produces an abnormally high voltage. As a result, an abnormal corona discharge occurs resulting in leakage, deterioration or break of the photoconductor and break of peripheral circuits of the high voltage transformer. While such accidents may be prevented when the high voltage transformer has an output limiting function, the open loop condition may not be noticed in that case and the operation may be continued while an excessive charge is maintained in the drum.